Internal combustion engines of automobiles are generally provided with rocker arms that pivot or swing about hydraulic tappets (lash adjusters) as hydraulically operated portions, camshafts that are rotated to rock the rocker arms, and intake and exhaust valves that are driven by these rocker arms.
To supply oil to the lash adjusters, pressurized oil is introduced from an oil pump that is driven upon start of the engine, into oil passages that lead to the lash adjusters. When the engine is stopped, however, the oil pump is stopped, and the surface of the oil in the oil passage is lowered. If the engine is restarted in this state, the oil pump is driven again, but the supply of the oil to the lash adjusters is delayed because of the lowered level of the oil in the oil passage. In the case where air is mixed into the oil in the oil passage, the air enters the lash adjusters, and causes undesirable noises during operation of the lash adjusters.
One type of oil supply device has been proposed, in which an oil inlet portion of an oil passage at which oil is introduced into the passage is located at a higher position than the other portion of the oil passage, as disclosed in Laid-open Publication No. 61-173706 of Japanese Utility Model Application. In this conventional oil supply device, the oil is prevented from being discharged from the oil passage even when the engine is stopped, and the oil can be readily supplied to suitable portions of the engine without delay when the engine is restarted.
In the conventional oil supply device as disclosed in the above-identified publication No. 61-173706, an air vent hole is formed at the oil inlet portion of the oil passage, so that air can be removed or discharged through the air vent hole at the time when the oil is introduced into the oil passage. Therefore, even if air is contained in the oil to be supplied to the oil passage, the air is removed in advance upon supply of the oil, and oil that is free of air is supplied to the oil passage. Consequently, air is prevented from entering the lash adjusters to which the oil is supplied.
In the conventional oil supply device as described above, however, air is removed or ejected at the time when the oil is supplied to the oil passage. Therefore, air, that was not removed at the oil inlet portion, is undesirably introduced into the oil passage. In the case where the oil mixed with air is supplied to the oil passage, it is difficult to remove the air from the oil after it enters the oil passage, and the air remains in the oil passage. Thus, in the conventional oil supply device, air may not be surely removed or ejected from the oil passage, and undesirably enters the lash adjusters, thus causing problems during operations of the lash adjusters.